1. Field of the Invention
The present invention relates to a winding arrangement for a power transformer of the autotransformer type, and specifically to a winding arrangement for an autotransformer having the same polarity impedance series and tertiary windings as defined by a three-terminal network equivalent of said autotransformer.
2. The Prior Art
In the transmission and/or distribution of electric power, it is normal practice to interconnect two or more power transformers in order to conveniently and efficiently deliver electric power. Power transformers used in interconnect systems are normally of the three-phase type; however, single-phase transformers can also be used in such interconnect systems. An autotransformer, in either a single or three-phase configuration, is one type of power transformer that is utilized in electric power transmission and/or distribution interconnect systems. A single-phase autotransformer is a well known device having its two basic windigs, which are normally designated series and common, magnetically coupled and connected together in electrical series. By contrast, in the normal two-winding transformer the individual windings are magnetically coupled but are electrially isolated.
Interconnected as well as non-interconnected autotransformers are sometimes exposed to phase-to-ground fault conditions. A phase-to-ground fault is, in effect, an excessively low impedance between a phase of a power transformer and a low potential point, or ground, to which power transformers are sometimes subjected for any number of reasons. One very common reason is as a result of an electrically conductive object making an electrical connection between an autotransformer phase and ground. Another very common reason is as a result of an internal transformer failure causing the aforementioned excessively low impedance to appear between an autotransformer phase and ground.
When a phase-to-ground fault occurs, one or more interconnected autotransformers will attempt to deliver an extremely large amount of power to such a fault, and will be successful in doing so if corrective measures are not promptly initiated. Failure to promptly initiate corrective measures will normally result in excessive autotransformer damage. The normal corrective measure in such a situation would be to remove the faulty autotransformer from the interconnected system of autotransformers. One way to electrically remove such an autotransformer is by means of a device called a ground fault relay. The function of this device is to disconnect an autotransformer, electrically, when the autotransformer is subjected to phase-to-ground fault conditions. This type of device is well-known and its operation is well understood in the electric power transmission and/or distribution field. If a ground fault relay is to operate properly when used with an autotransformer, it is essential that the polarity of the impedance of the series and the tertiary windings of said autotransformer be the same, especially during phase-to-ground fault conditions. A tertiary winding of an autotransformer is an electrically isolated auxiliary winding that normally has limited power handling capability relative to other autotransformer windings. The polarity of the impedance of the series winding and the tertiary winding of an autotransformer will, by definition, be the same if the three-terminal network equivalent of these windings have the same algebraic sign.
In the past, achieving the above-mentioned desired polarity impedance of autotransformer series and tertiary windings has met with varying degrees of success. One previously used way was to merely optimize the design parameters of concentrically positioned and radially superposed tertiary, common and series autotransformer windings. The tertiary winding in such an autotransformer was the innermost winding and the series winding was the outermost winding. Another previously used technique was to concentrically position and radially superpose tertiary, common and series windings around a magnetizable core. Approximately half of the tertiary winding was adjacent the core while the other half of same was the outermost winding. The common and series windings were located between the tertiary winding with the common winding being closer to the core than the series winding. While both of these windings arrangements provided the desired impedance polarity, such winding arrangements produced unfavorable voltage gradients between windings or between a winding and the transformer core. The present invention, among other things, substantially reduces this unfavorable voltage gradient.
Another method of limiting damage to an autotransformer from a phase-to-ground fault was to place an external reactor on each autotransformer phase in order to place an upper limit on the magnitude of fault current. The disadvantages of such an arrangement are that external reactors are costly and require additional space. An autotransformer incorporating the present invention would not require such an external reactor.